Microwave devices using garnet-films have widespread application and are, in particular, used in radar systems, electronic warfare, telecommunication satellites, avionics and related instrumentation.
Such systems increasingly require higher operational frequencies and wider bandwidths.
This has been an incentive in the search for new techniques and technologies for signal processing. A relatively new analog technique useable in the microwave field has been developed, based upon magnetostatic wave (MSW) propagation in Yitrium-Iron garnet (YIG) class ferrimagnetic films (Y.sub.3 Fe.sub.5 O.sub.12 and cationic replaced derivatives thereof such as LaGa:YIG) having low losses, achieved by liquid phase epitaxy (LPE). MSW are slow electromagnetic waves which propagate at microwave frequencies in magnetically based ferrites.
Their salient parameters are:
Power level: orders of magnitude of .mu.W to mW PA1 Frequency range: 0.5 to 20 GHz PA1 Velocity range: 3 to 300 mm/.mu.sec PA1 High coupling efficient PA1 Propagation losses: as low as 0.02 dB/nsec PA1 Bandwidth: up to 1 GHz PA1 Dynamic interval: +50 dB PA1 Delay times: 1 to 1000 nsec PA1 Magnetic field bias: 0.1 to 10 KOe PA1 .gamma.(H.sub.o +2.pi.M.sub.s), (low), and PA1 .gamma..sqroot.H.sub.o (H.sub.o +4.pi.M.sub.s), (high), respectively, PA1 .gamma..sqroot.H.sub.o (H.sub.o +4.pi.M.sub.s) (low) and .gamma.H.sub.o (high). PA1 K.sub.y =n.sub.y /1.sub.y PA1 K.sub.z =n.sub.z /1.sub.z
MSW can be tuned to all microwave frequencies by varying the magnetic bias field (H.sub.o). Depending upon the orientation of H.sub.o, with respect to the sample plane, it is possible to generate magnetostatic surface modes (MSSW) and/or magnetostatic forward (MSFVW) and magnetostatic backward (MSBVW) volume modes, as will be described below.
Devices using MSSW have cutoff frequencies given by
where .gamma. is the gyromagnetic constant (.gamma.=2.81 MHz/Oe for YIG) and ambient temperature).
Devices operating in volume modes have respectively the following cutoff frequencies
A typical ferrimagnetic resonance (FMR) spectrum (discussed further below) on a square chip (2.3.times.2.3 mm.sup.2) made of YIG, at the fixed frequency of 9.26 GHz, where field H.sub.o is parallel to the film plane has been illustrated in the drawing; in such configuration both MSSW and MSBVW modes are excited simultaneously. For each mode the respective wave numbers (ny, nz) are indicated, related to wave vectors parallel to the film plane by the following:
where 1.sub.y, 1.sub.z are the chip's lateral dimensions.
As a consequence of the above, by varying the bias field H.sub.o, the frequency and the propagation vector K, it is possible to have a complete set of curves characterizing the dispersion of the various magnetostatic modes. The most direct consequence of this fact is the possibility of delaying a microwave signal which has been injected, in an opportune manner, into the epitaxial film by means of microstrip transducers laid directly onto the magnetic film or, better still, onto an alumina substrate which can then be faced with the magnetic film.
Other possible applications, thanks to the low magnetic losses, are in the field of filters, resonators and oscillators.
In some circuit applications (such as filters) it may be necessary to attenuate or to suppress single modes or entire regions of the magnetostatic band.